Spiral antenna with spiral reflecting cavity



A ril 28, 1964 3,131,394

M. 5. WHEELER SPIRAL ANTENNA WITH SPIRAL REFLECTING CAVITY Filed Jan. 22, 1962 INVENTOR. MYRON 5. WHEELER gfm AGENT qw l- A TTORN E Y United States Patent 3,131,394 Si IRAL ANTENNA WlTH SPIRAL REFLECTHNG QIAVITY Myrcn S. Wheeler, Baltirnere, Md, assignor, by mesne assignments, to the United States at America as represented by the Secretary of the Navy Filed Ian. 22, 1962, Ser. No. 167,984 4 Claims. ((Il. 343-895) This invention relates in general to microwave antenna systems, and in a preferred embodiment to spiral antenna systems that include a reflector element.

The use of reflectors for concentrating the radiation from an antenna into a beam is well known. Various forms of reflectors are utilized, such as parabolas and troughs, with the antenna being placed in front of the concave side of the reflector at its focal point. Such a system is inherently of narrow band width as compared with the band width of the spiral antenna itself. As a result, when a metallic reflector is used to limit the spiral radiation to a mono directional pattern (as contrasted with a normal bidirectional pattern) the useful band width is greatly reduced. The problem is especially acute in the case of a spiral designed to produce a high degree of circularity of its radiated energy.

The present invention greatly reduces the depolarization associated with an antenna system in which the antenna is of spiral configuration. This is accomplished by redirecting much of the radiation that is emitted in undesired directions, so that said redirected radiation reinforces the beam in a predominantly additive phase relationship independent of the applied frequency. To attain this objective, the spiral antenna is positioned in front of the convex side of a conical reflector, and a spiral battle device with reflective surfaces is interposed between the antenna and the conical reflector. The result is a single reinforced beam from the antenna system having a rather broad pattern (about 80 width) of highly circularly polarized radiation.

An object of the present invention, therefore, is to provide a simple means for redirecting antenna radiations which would otherwise travel in undesired directions and hence not contribute to the useful output of the antenna.

Another object is to provide a wide band antenna system in which the beam is reinforced by energy which would produce narrow band width when utilizing conventional structures.

A further object is to provide an efficient beam antenna system incorporating such a spiral antenna.

Other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following detailed description of a preferred embodiment of the invention as illustrated in the accompanying drawing in which:

FIG. 1 is a perspective view of an antenna system constructed in accordance with the present invention;

FIG. 2 is an isometric view of two spiral baflles supported upon the conical reflector of FIG. 1;

FIG. 3 is a front elevational view of the reflector showing the disposition of two radiating arms of the spiral antenna of FIG. 1 and the two spiral baflies of FIG. 2; and

FIG. 4 is a sectional elevation taken along line IV-IV of FIG. 3.

Referring now to the drawing, wherein like reference numerals designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a preferred embodiment of a spiral antenna system ltl designed in accordance with this invention. Numeral 11 represents an enclosed radio frequency transmitter device which may be of conventional design and hence has been illustrated in schematic fashion. Support means 13 contains antenna feed means (not shown) for delivering a pushpull input from transmitter device 11 to a spiral antenna 15. The spiral antenna is supported perpendicular to the axis of conical reflector 17 by any conventional means (not shown). The reflector is attached either fixedly or rotatably to support means 13, also in a conventional manner. A spiral baflle device 19 is fixedly attached to reflector 1'7, with the forward edges of said batfle device in the plane of, but not in contact with, spiral antenna 15.

Spiral baflle device 1% consists of two interwound spaced spiral baflies 21, whose ends are diametrically opposed to each other, as illustrated by FIG. 2. Said spiral battles are strips whose surfaces, at least, are highly reflective, therefore they may be made of metal, aluminum for eX- ample. Each baffle is fastened at one edge 23 to conical reflector 17 in a manner to establish substantially continuous contact therebetween. Since high reflectivity is the purpose of both the reflector and the baflles, both members may be made of the same material and integrally constructed.

When viewed from the front, as shown in FIG. 3, the spiral antenna consists of two radiating arms 25 that, with the two spiral baflies 21, appear as a four-arm spiral. The antenna illustrated is a typical spiral antenna, composed of two radiating arms disposed in a plane, these radiating arms originating at a center region and spiralling outwardly for a selected radial distance. The corresponding ends of the radiating arms terminate at points which are diametrically opposed to each other, as shown. While baffles 21 are preferably in the form of strips, antenna radiating arms 25 may be of any conventional nature, such as Wire rods, metal strips, or metal tubes, the last mentioned form being used in the embodiment shown.

The configuration of the spirally-interwound antenna radiating arms may vary, e.g., it may be Archimedean, exponential, etc., and the configuration of the baflies and the slope of reflector 17 are consequently chosen in accordance with the particular form of spiral utilized. In the embodiment of the invention illustrated, both the radiating arms and the baflles are in the form of Archimedean spirals, with the baffles being centered between the adjacent radiating arms, as best shown in FIG. 3 of the drawing.

FIG. 4 further illustrates the spatial relationship between conical reflector 17, spiral baflles 21 and antenna radiating arms 25. Antenna feed means 27 delivers input energy to the radiating arms from transmitter device 11. This feed means is shown passing through an aperture 29 in the apex of the reflector, the aperture also being suitable for accommodating a support means (not shown) for the radiating arms in the event means 27 does not provide adequate support. Spiral baffles 2l1 extend from the outer conical reflector surface to the plane containing the radiating arms, the baffles being perpendicular to said plane in this embodiment. As can be seen in FIG. 4, the Width of each bafile progressively decreases as it approaches the apex of the reflector. It is apparent that all back radiations are intercepted by the reflector and the baffles. The highly reflective surfaces of these members reflect said radiations forward; when the reflected radiations reach the plane of the spiral antenna arms they can only be traveling in the same general direction as those forward radiations being then emitted by said antenna. The baflies 21 are logarithmic in nature and do not have frequency-sensitive lengths.

In order for the reflected waves to reinforce the forward radiations, the reflected waves should reach the plane of radiating arms 25 in .a predominantly in-phase relationship. This is effected by control of the wave length of the radiations and the spacing between the radiating arms and reflector 17. Since methods for causing said spiral radiating arms to radiate predominantly at certain corresponding locations are well known to those skilled in the art, the problem is simplified. In the embodiment shown in FIG. 4, if a cavity depth of A/n is desirable in region Z of the spiral (where the spiral is radiating predominantly at a specified frequency) then doubling the frequency cuts the diameter ZZ in half and reduces the distance to the reflector behind it in half. Thus the assembly is self-compensating and the radiation pat-tern remains unchanged.

It is apparent that one of the spiral baflies 21 can be eliminated, leaving the two antenna radiating ohms between successive baflie surfaces along any given diameter. Other variations of the invent-ion to suit particular antenna applications are contemplated; for example, forms of antennas other than spiral may be accommodated by altering the configuration of the baffle device to conform thereto. Likewise, by changing the angular relationship of the baffle device to the reflector a change can be achieved in the pattern of reflected radiation. Changes in the configuration of the reflector and/or its spacing from the radiating antenna element, for example, are advantageous when the invention concept is employed with other types of antenna systems whose emissions cannot be suitably redirected by conventional reflectors.

As disclosed by the foregoing, this invention provides means for utilizing antenna radiations traveling in undesired directions to reinforce the radiations in the desired direction. For those antennas whose radiating surfaces are placed at specific wave length distances to a reflector and thus become narrow band in application, the invention provides a unidirectional antenna system more eflicient than heretofore possible. In addition, the reflector need not be much larger than the radiating elements of the antenna used, thereby reducing the size of the reflector required.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. A unidirectional antenna system comprising: an active antenna element in the form of two electricallyinterconnected arms disposed in coplanar relationship, each of said arms having the configuration of a spiral the radius of which progressively increases outwardly from a central point, the said two arms being electrically interconnected at such central point; means for energizing said antenna element; a conical reflector, said active element being mounted adjacent said reflector so that said element is in front of, and symmetrically disposed with respect to, the apex of the convex side of said reflector, and in a plane normal to the axis of revolution of the latter; a pair of baffle strips respectively and equidistantly disposed in alternating fashion between the arms of said element, each baflie strip having a spiral form similar to that of the antenna arms, each of said baffle strips being composed of material capable of reflecting at least a portion of the energy radiated from said antenna arms when said system is in operation; and means for mounting said baflies upon said reflector whereby the baffles extend in a direction normal to the plane defined by the antenna arms, with corresponding edges of the baiiies in substantially continuous contact with the surface of the reflector, and the opposite corresponding edges of the bafiies in the plane of, but spaced from, the arms of said antenna.

2. A unidirectional antenna system comprising: a spiral antenna; a source of energy to be transmitted; antenna feed means connecting said antenna to said source 'of energy; a conical reflector; means for mounting said operation; means for mounting said baflie strip upon said reflector so that one edge of said strip is in contact with the reflector, and the surfaces of both the baflle strip and the reflector act to redirect energy emitted by the spiral antenna, whereby such redirected energy reinforces energy directly emitted by said antenna; and means for supporting and positioning said spiral antenna, said reflector and said baffle so that when energy is received by said spiral antenna over said feed means from said source, the energy transmitted from said system will have substantially only a single direction of propagation.

3. In a unidirectional antenna: an active radiating unit in the form of a pair of oppositely-disposed electricallyinterconnected arms spiralling outwardly in a plane essentially normal to the desired direction of propagation of energy from said antenna; a conical reflector having the configuration of a surface of revolution, the axis of which is parallel to the said direction of energy propagation and normal to the plane defined by said arms, the apex of said conical reflector lying generally in the plane of said antenna arms and the open end of said reflector extending in a direction from said plane opposite to that in which energy is propagated from said unit when said antenna is energized; and a pair of baffles in the form of strips of material capable of reflecting radiant energy impinging thereupon, said strips spiralling outwardly from the region where said pair of arms are electrically-interconnected and being respectively interposed therebetween so as to alternate therewith considered angularly with respect to said point of arm interconnection, corresponding edges of said strips being supported on and positioned by the surface of said reflector so that the remaining edges of said strips lie generally in the plane defined by the said interconnected arms, the width of each strip of said pair thus increasing radially outwardly from the region where said pair of arms are electrically interconnected.

4. In a broad band antenna system designed to propagate energy in a single direction, said system having an active antenna element designed to radiate energy over a relatively broad band of frequencies, different portions of said antenna element being active at different frequencies of operation of said system, the improvement residing in the combination of a baflie means and a reflector, said baffle means being composed of material capable of reflecting at least a portion of any radiant energy impinging thereupon, said baflie means being mounted substantially continuously with said reflector and in generally angular relation to said active antenna element, the respective reflective surfaces of said baffle means and said reflector being disposed in an angular relationship to one another and with the reflecting surface of said reflector being disposed in a varying spatial relationship to said active antenna element such that each portion of said reflector is spaced one-quarter wavelength from the particular portion of said antenna element which is active at the frequency of operation of said system at a given instant of time, whereby the combination of baffle means and reflector intercepts radiant energy from said active antenna element which is emitted in direction other than said single direction and redirects at least a portion of such intercepted energy into a path generally coinciding with that followed by the energy propagated in said single direction, thereby reinforcing such last mentioned energy.

References Cited in the file of this patent UNITED STATES PATENTS 2,455,403 Brown Dec. 7, 1948 2,471,284 Rea May 24, 1949 2,856,605 Jacobsen Oct. 14, -8 2,863,145 Turner Dec. 2, 1958 3,019,439 Reis et al. Jan. 30, 1962 3,055,003 Marston et a1. Sept. 18, 1962 

1. A UNIDIRECTIONAL ANTENNA SYSTEM COMPRISING: AN ACTIVE ANTENNA ELEMENT IN THE FORM OF TWO ELECTRICALLYINTERCONNECTED ARMS DISPOSED IN COPLANAR RELATIONSHIP, EACH OF SAID ARMS HAVING THE CONFIGURATION OF A SPIRAL THE RADIUS OF WHICH PROGRESSIVELY INCREASES OUTWARDLY FROM A CENTRAL POINT, THE SAID TWO ARMS BEING ELECTRICALLY INTERCONNECTED AT SUCH CENTRAL POINT; MEANS FOR ENERGIZING SAID ANTENNA ELEMENT; A CONICAL REFLECTOR, SAID ACTIVE ELEMENT BEING MOUNTED ADJACENT SAID REFLECTOR SO THAT SAID ELEMENT IS IN FRONT OF, AND SYMMETRICALLY DISPOSED WITH RESPECT TO, THE APEX OF THE CONVEX SIDE OF SAID REFLECTOR, AND IN A PLANE NORMAL TO THE AXIS OF REVOLUTION OF THE LATTER; A PAIR OF BAFFLE STRIPS RESPECTIVELY AND EQUIDISTANTLY DISPOSED IN ALTERNATING FASHION BETWEEN THE ARMS OF SAID ELEMENT, EACH BAFFLE STRIP HAVING A SPIRAL FORM SIMILAR TO THAT OF THE ANTENNA ARMS, EACH OF SAID BAFFLE STRIPS BEING COMPOSED OF MATERIAL CAPABLE OF REFLECTING AT LEAST A PORTION OF THE ENERGY RADIATED FROM SAID ANTENNA ARMS WHEN SAID SYSTEM IS IN OPERATION; AND MEANS FOR MOUNTING SAID BAFFLES UPON SAID REFLECTOR WHEREBY THE BAFFLES EXTEND IN A DIRECTION NORMAL TO THE PLANE DEFINED BY THE ANTENNA ARMS, WITH CORRESPONDING EDGES OF THE BAFFLES IN SUBSTANTIALLY CONTINUOUS CONTACT WITH THE SURFACE OF THE REFLECTOR, AND THE OPPOSITE CORRESPONDING EDGES OF THE BAFFLES IN THE PLANE OF, BUT SPACED FROM, THE ARMS OF SAID ANTENNA. 